In some ways, jet aircraft symbolize a traditional conflict between technological progress and individual rights and privacy. On one hand, development of jet powered transport aircraft has been a giant step forward in improving airline convenience by shortening the transit time between cities and countries. On the other hand, because the turbine engines used by such aircraft have been noisy, especially during takeoff and landing, jet aircraft annoy people in communities located in the vicinity of airports. As a result, there is a continuing worldwide concern about the noise produced by turbine engines. Sensitive to this concern, aircraft and engine manufacturers have developed, and are continuing to develop, noise-suppression devices adapted to reduce engine noise. (As used herein, turbine engines include turbo-prop, turbo-fan and turbojet engines.)
Most modern aircraft are powered by fan-jet engines, which have a number of noise sources. These noise sources contribute to various noises having different frequency characteristics. As turbine engine noise has become better understood, various types of noise have been given identifying names. Of the identified types of noise, the major contributors to noise during takeoff and landing are denoted buzz saw, forward generated fan and aft generated fan noise. The name buzz saw is generally associated with noise covering a broad frequency band, for example from 50 to 2400 Hertz, produced a series of tones 50 to 60 Hertz apart. The names forward and aft generated fan noise are generally associated with discrete tones covering narrow frequency ranges that are primarily emitted from the front and rear of the engine respectively. For example, foward generated fan noise may be a discrete tone having a center frequency of about 2,200 Hertz, plus harmonics thereof. Attempts have been made to attenuate the aforementioned types of noises by lining selected regions of the inlet and fan air duct of fan-jet engines with various types of noise absorbing acoustic panels. In the past, the principal noise absorbing acoustic panels used in such turbine engines have been single or multilayer cellular acoustic panels.
While bulk absorber panels have been tested for use in turbine engines, for various reasons previous designs have been found to be unacceptable for use in production engines. More specifically, the bulk absorber panels previously tested in jet engines have excellent broadband noise attenuation characteristics in the range from about 50 to about 4,000 Hertz. A typical bulk absorber panels includes a layer of bulk absorber material mounted between a back sheet and a perforated face sheet. The perforated face sheet allows sound to enter the bulk absorber material. The face sheet, in combination with the back sheet, gives the bulk absorber panel a limited amount of structural stiffness. One disadvantage of bulk absorber panels is that when they are located in a high speed airflow region, such as the inlet and fan air duct of a fan-jet engine, the openings in the face sheet allow air and contaminants to impinge on the bulk absorber material. Contaminants can change or destroy the acoustic properties of the bulk absorber material. Turbulence and high speed air flows in close proximity to the bulk absorber material can cause it to disintegrate and be carried out of the panel through the openings in the face sheet. Not only are the noise attenuation characteristics of the bulk absorber material destroyed by this process, the disintegrated bulk absorber material can be harmfully sucked into the high pressure sections of the engine. Because of this susceptibility to break down, bulk absorber linings have found little use in production aircraft engines.
Another problem with bulk absorber panels is that the structural stiffness provided by only a perforated face sheet and a back sheet is inadequate to permit such panels to be effectively utilized in turbine engines. More specifically, because of the limited structural stiffness of bulk absorber panels, many structural members are required to ensure that the panels are structurally secure within the jet engine inlet and fan air duct. The many structural members needed to support the bulk absorber panels have the obvious disadvantage that they add considerable unwanted weight to the engine. In addition, the structural members have the disadvantage that they result in a significant loss of effective acoustical area and, thus, loss in the amount of noise that can be absorbed or attenuated. That is, structural stiffeners do not, of course, have noise absorbtion qualities. Rather, they form hard noise reflecting surfaces.
In an attempt to increase the stiffness of bulk absorber panels, bulk absorber material has been mounted in the cells of a honeycomb layer mounted between a back sheet and a perforated face sheet. (See U.S. Pat. No. 3,095,943 entitled "Acoustical Structure" and U.S. Pat. No. 3,380,552 entitled "Acoustical Panel With Honeycomb Core and Ventilation Passages".) The disadvantage of mounting bulk absorber material within the cells of the honeycomb layer is that trapped water and other contaminants can accumulate within the cells and add considerable weight to the jet engine. Moreover, the water trapped in the honeycomb cells can freeze and melt because of the temperature extremes experienced by a jet engine as an aircraft changes altitudes between takeoff and landing. Such freezing and melting can cause both the honeycomb layer and the bulk absorber to break up or fracture. Additionally, trapped water and other contaminants reduce the noise attenuation capabilities of bulk absorber materials and, in the extreme, may totally prevent noise suppression. Further, the installation of bulk absorber material in individual honeycomb cells is extremely difficult; and, such installation severely restricts the type of bulk absorber material that can be used.
Because of the disadvantages associated with current methods of using bulk absorber materials in turbine engines, more recent noise reduction efforts have been directed to the development of lightweight, multilayered cellular acoustic panels. In U.S. Pat. No. 3,670,843 entitled "Sandwich Structure", for example, two cellular layers are separated by a porous layer; and, an impervious layer is attached to the outer face of one of the cellular layers. In other multilayered cellular acoustic panels a first layer of cellular material (often a honeycomb layer) is mounted between (and bonded to) an impervious back sheet and a perforated separation sheet; and, a second layer of cellular material is mounted between (and also bonded to) the perforated separation sheet and a perforated face sheet. (See U.S. Pat. No. 3,439,774 entitled "Sound Energy Absorbing Apparatus", U.S. Pat. No. 3,640,357 entitled "Acoustic Linings", and U.S. Pat. No. 3,948,346 entitled "Multi-Layered Acoustic Liner".)
From an acoustical standpoint, the cellular layers of cellular acoustic panels are generally designed to attenuate noise lying within a specific narrow range of frequencies, rather than a broad range of noise frequencies. The volume of the cells, particularly the cell's depth dimension, determines the resonant frequency of the cells and limits the narrow range of frequencies that will be attenuated. It is precisely this characteristic that forms the major disadvantage of prior art multilayered cellular acoustic panels. More specifically, the major disadvantage of the multilayered cellular acoustic panels is that their noise attenuation effectiveness is limited to one or more relatively narrow ranges of frequencies. Noise at frequencies lying outside of the narrow range or ranges of frequencies is only attenuated slightly, if at all. More specifically, as with an electronic circuit filter having a frequency rejection band, the farther the frequency of a signal (e.g., noise) is away from the band, the less the signal is rejected (e.g., absorbed). It is for this reason that prior art cellular acoustic panels are usually multilayered. That is, such panels must include cells of differing depth if a plurality of discrete tones at widely separated frequencies are to be significantly attenuated. While differing cellular size can be achieved by including cells of differing depth in a single layer, such a layer is somewhat difficult to produce. More importantly, since the magnitude of a particular signal that is to be attenuated is a direct function of the number of cells designed to attenuate at the frequency of that signal, reducing the number of cells of a particular depth reduces the magnitude of the attenuation of signals related to that cell depth. It is for these reasons the prior art cellular acoustic panels include a plurality of layers of cells, rather than a single layer.
Another problem will multilayered cellular acoustic panels is that contaminating liquids, such as water, can enter such acoustic panels through the porous face sheets and core separation sheets; and, become trapped in the cells. Trapped liquids have the same three disadvantages in cellular acoustic panels that they have in bulk absorber panels wherein the bulk absorber is mounted in a honeycomb layer: first, they reduce the noise attenuation attributes of the panels; second, they add unwanted weight to the engine; and, third, freezing and melting of the liquids as the altitude of the aircraft changes may fracture the acoustic panels. To alleviate the problems associated with water entrapment in multilayered cellular acoustic panels, drain slots are typically formed in adjacent walls of the cells. However, drain slots have the disadvantage that they have an unpredictable effect on the attenuation capacity of an acoustic panel.
Another disadvantage of multilayered cellular acoustic panels is that they are generally expensive to manufacture. It is extremely difficult to determine if an adequate bond has been created between the surface of the cells and the central perforated sheets (septum) that separate adjacent cellular layers. A visual inspection is impossible because the septum sheet is hidden between the cellular layers; and no reliable test equipment is available. Additionally, the acceptable percent of open area in the perforated sheets separating cellular layers is low and must be held within fairly narrow tolerances. For example, in one type of cellular acoustic panel suitable for use in turbine engines, the allowable percent of open area in the perforated sheet separating two cellular layers must lie between 3% and 41/2%. It is extremely difficult to meet this requirement because, when bonding the layers to the perforated separating sheets, it is almost impossible to predict how many openings will be closed by the interfaces of cell walls and the bonding material. That is, bonding resin cannot be applied and cured in a manner that will prevent some of the perforated openings from being closed; and, it is impossible to accurately predict how many openings will be closed. As a result, multilayered cellular acoustic panels must be constructed as carefully as possible to ensure that adequate bonding exists between the cellular layers and the perforated sheet separating the layers. Then they must be tested to ensure that the percent of open area in the perforated separation sheets is within tolerances. Multilayered cellular acoustic panels that do not meet both of these criteria must be rejected. When many panels fail to meet both criteria and must be rejected, production costs are significantly increased.
It is therefore an object of this invention to provide a new and improved acoustic panel.
It is also an object of this invention to provide a new and improved acoustic panel that is suitable for use in turbine engines.
It is another object of this invention to provide a new and improved acoustic panel suitable for use in turbine engines that can effectively attenuate noise over a broad range of frequencies.
It is a further object of this invention to provide a new and improved acoustic panel suitable for use in turbine engines that has maximum noise attenuation characteristics over selective frequency ranges and significant noise attenuation characteristics over the entire range of frequencies produced by a turbine engine.
It is a further object of this invention to provide a new and improved acoustic panel suitable for use in turbine engines that is inexpensively manufactured and easily tested, and that is able to stand up to the severe environments existing within a turbine engine.